1. Field of the Invention
The present invention relates to an FM antenna and more particularly to an FM wave receiving antenna.
2. Prior Art
In prior art, metal rod type antennas and linear folding type antennas have been used for automobile FM wave antennas and demountable FM broadcast receiving antennas. When antenna gain should be considered, antennas having wavelengths of 1/2 to 1/4 are most preferable, because the receiving level is abruptly reduced if the antenna has shorter wavelength. For this reason, antennas of 80 cm or longer have been mostly used in Japan.
In antennas for frequencies above the VHF band, the rod type antenna and folding line type antenna have 1/4 wavelength or multiples thereof in whole numbers. Such antennas are usually too long. Accordingly, shortening devices such as coils are put in the antenna. However the shortening of the length results in sacrificing some gain. Thus, the shortening of antenna length involves some limitations.
Plate form antennas are occasionally used as electrostatic antennas. However, in order to obtain a sufficient gain, metal plates of as large surface area as 300 cm.sup.2 are required.
Furthermore, in automobiles, it is preferred not to have any projected objects on the vehicle body in view of design preference and in order to avoid noises which they would make when the car is in motion. No conventional antennas could have met these demands. Antennas for indoor equipment and those incorporated in receivers, on the other hand, are formed flat so that they lay against the attached surface of the equipment, and receivers are required to be as unobtrusive as possible for esthetic reasons and harmony of indoor environments. However conventional antennas are not satisfactory in this area, either.